Although choosing frac sand can reduce completions design cost initially, there can be short-term and long-term issues: Proppant flowback, proppant embedment, and decreased fracture conductivity, in addition to fines, can negatively affect hydrocarbon production.
What are fines?
Fines are generated by crushing of proppant, by erosion of sand on the fracture face during treatment, by incompatibility between the formation and the fracturing fluid, or by embedment of the proppant into the fracture face (Coulter and Wells, 1972).
Sand, which is strong, cost effective, and available, will crush in deeper wells and tends to flow back or embed. Bauxite is stronger than sand, chemically and slightly deformable, but does flow back and can erode valves, chokes, tubular goods, and even the wellhead. In some formations, bauxite’s high hardness spalls rock or generates formation fines detrimental to the well’s production (Sinclair, Graham, and Sinclair, 1983).
Fines reduce conductivity
Because the flow path to the wellbore can be very turbulent, fluids in nondarcy flow transport fines, including crushed sand, to the near-wellbore area. Sometimes, fines plug the proppant pack also near-wellbore. Fines can cause severe well production loss by lowering conductivity. In the landmark 1972 paper cited above, the authors determined at 3,500 psi closure stress, 20/40 raw sand at 1 lb/sq ft with 5% added fines can reduce conductivity up to 60%.
The best completion technology cannot overcome reservoir damage by fines movement and plugging, especially in depleted wells. An operator could refrac the well, but that would incur workover expense.
Resin-coated sand minimizes fines
While high-temperature water or brine will attack fresh siliceous (Northern White sand) surfaces, the resin surface is nonreactive to any reservoir fluid system. (Sinclair, Graham, and Sinclair, 1983). Additionally, also according to laboratory testing, frac sand can lose 80% conductivity at 350º F reservoir temperature. At extremely high closure stress, fines migration is minimized because the resin is securely bonded to the sand grain. Fracture conductivity improves with minimized fines.
Resin-coated sand produces a better well
Resin-coated sand’s durable outer layer reduces fines generation and movement. With significantly decreased fines, resin-coated sand offers greater conductivity and robust flow paths that allow operators to optimize their hydraulic fracturing investment and produce a better well.